The present invention relates to a method of operating fuel cell system which uses a fuel cell for power generation and a supply of heat.
In a known fuel cell system, a hydrogen-rich gas supplied as fuel gas and air supplied as oxidizing gas are allowed to react to each other, thereby generating electricity and heat within the fuel cell. The fuel gas and oxidizing gas are supplied to the fuel cell after respectively humidified by a humidifying means. One example of the humidifying means for the fuel gas and oxidizing gas is a bubbler which performs humidification by letting the fuel gas and oxidizing gas pass through hot water heated by a heater (see Japanese Patent Publication Kokai No. 7-288134). This fuel cell system, however, suffers from a decline in energy efficiency because the bubbler consumes energy for heating water.
Another known humidifier is such that moisture (water vapor) contained in exhaust air (off gas) discharged from the air pole side of the fuel cell is transferred to air to be supplied to the air pole side of the fuel cell through a vapor transmitting film, thereby humidifying the supply air (see Japanese Patent Publication Kokai No. 6-132038). This humidifier is designed to carry out humidification by use of exhaust air of high temperature, thereby saving energy required for the humidification. This humidifier has revealed such a disadvantage that the gas to be humidified (which is, herein, the supply air to be humidified) cannot be humidified to a dew point temperature higher than that of the humidifying gas (which is, herein, the exhaust gas serving as a moisture supply source). In addition, if the gas is humidified to a high dew point temperature, vapor transmitting films having large film area become necessary, leading to an increase in the scale of the humidifier. Therefore, downsizing of the fuel cell system becomes difficult.